After a nearly daylong power outage upended life for millions across Spain and Portugal, the authorities in both countries are still searching for the cause of the disruption.
Some of Spain’s top officials convened on Wednesday morning to discuss the blackout, which halted critical infrastructure starting Monday afternoon for nearly 18 hours in some areas.
Power was almost entirely restored by Tuesday in Spain and Portugal, as well as in southern France, where households in the Basque region were affected.
The cause remains under investigation.
Spain’s top security and defense officials met on Wednesday to discuss the blackout, with officials saying they were considering a range of possibilities. The Spanish government has asked European regulators and various domestic agencies to investigate what happened.
On Tuesday, Eduardo Prieto, director of services for Spain’s national power company, Red Eléctrica, said that there were no “definitive conclusions” about the reasons for the outage, although he and other officials ruled out a cyberattack.
Prime Minister Pedro Sánchez of Spain said that a committee of technical experts from the European Commission would investigate the cause.
Some facts have emerged about how the blackout began.
Around noon on Monday, a high-voltage connection between France and Spain was interrupted, according to Kristian Ruby, secretary general of Eurelectric, a trade body that represents the European electricity industry. The power outage occurred just over 30 minutes later.
While that interruption would have been expected to be disruptive, it would not normally lead to a “system collapse” like the one that occurred on Monday, Mr. Ruby said. Some other complication would typically need to happen, “like a sudden outage at a power plant, a sudden development on the demand side,” he said. “Then you can have an incident like this.”
Hospitals, banks and travel were disrupted.
During the outage, there were widespread problems connecting to the internet and to phone networks across Spain and Portugal.
Hospitals in Spain were forced to run on generators. Portuguese banks and schools closed. Spain’s national rail company said that trains had stopped operating at all stations. Subways in several cities, including Valencia and Madrid, were halted. And people crammed into stores to buy food and other essentials as clerks used pen and paper to record cash-only transactions.
Temperatures at the time of the outage were higher than usual.
According to Spain’s meteorological agency, AEMET, temperatures across the country were between 60 and 70 degrees Fahrenheit around the time of the outage. By the end of the day, highs had climbed to between 82 and 87 degrees.
In a typical April, temperatures in Spain average around 64 degrees in central and northern regions and around 70 degrees in the south.
Asked whether the outage was connected to the heat, Bruno Silva, a spokesman for the Portuguese electricity supplier REN, was skeptical. “No, I hope not,” he said.
Attention has focused on Spain’s reliance on renewable energy.
The incident has raised questions about whether Spain’s rapid shift to renewable energy left it more vulnerable to outages.
More than half of Spain’s electricity came from renewable energy last year, putting it at the forefront of Europe’s transition to sources such as wind turbines and solar farms. That has led to lower electricity prices in Spain, but the rapid transition has also left the Spanish power grid — and Portugal, which it supplies energy to — prone to disruptions, analysts say.
Old-line power sources like gas turbines and nuclear plants are better at buffering the fluctuations that can arise with intermittent sources like wind and solar power. Experts said that it might have been easier for Spain to manage instability in its power grid if conventional power sources had a larger presence, and if Spain had invested more in grid infrastructure and electricity storage facilities, like batteries to provide backup power.
But officials in Spain have said they don’t believe there is a connection between the energy supply and the outage. Beatriz Corredor, the chairwoman of Red Eléctrica, told Spanish radio on Wednesday that it was “not correct to link the incident to renewable energy.”
Other European outages have halted daily life.
More than 50 million people in Italy were left in the dark for almost a full day in 2003 after a line between Switzerland and Italy was overloaded. It was considered the worst day of blackouts in the country since World War II.
In 2006, 10 million people in Germany were briefly without power after the northwestern part of the country’s power grid became overloaded.
And last year, much of the Balkans was without power for several hours during a heat wave in which temperatures soared to 40 degrees Celsius, or over 100 degrees Fahrenheit.
Nazaneen Ghaffar, Mike Ives and John Yoon contributed reporting.
Sara Ruberg covers breaking news and is a member of the 2024-25 class of Times Fellows, a program for journalists early in their careers.
Michael Levenson covers breaking news for The Times from New York.
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